In the printed circuit industry, photographic masks or stencils bearing a circuit pattern, are known as phototools. Such a stencil, which commonly is a diazo film or a gelatino/silver halide emulsion on a polyester base through which a photoresist can be exposed, provides an intricate, complex image representing an electrical circuit. The image often consists of many fine lines and junctions spaced closely together. During its use to make printed circuit boards, the phototool is placed face down on a photoresist layer and a contact print is made by exposing the photoresist to high intensity light through the phototool. In this way, a single phototool can be used to make hundreds of contact prints.
After processing, a phototool must be carefully inspected through a microscope to insure that there are no breaks in the fine lines of the image. The continued use of the phototool causes tiny scratches and abrasions on the phototool surface. The photoresists on which the phototool is placed, are usually laminated on sheet copper; small burrs or rough edges of the copper sheet can cause scratches as the phototool is transferred from one photoresist to the next. The phototool is frequently wiped with a soft cloth to make sure it is dust and lint free because small particles of dirt can cause scratching as they are wiped across the phototool surface. Because of this general wear and tear on the phototool surface during normal use, the phototool must be frequently reinspected (after 20 to 50 exposures) to insure line continuity. Depending on the size and the intricacy of the phototool, such microscopic inspections can take up to 2 to 3 hours.
The fact that phototools are vulnerable to scratching and that abrasion is a serious problem during the normal use of a phototool is confirmed by the large number of patents claiming diazo-type emulsions having toughness and resistance to abrasion that are said to be useful in circuit board technology.
Protective overcoats for photographic elements are well known, and a wide variety of products have been proposed in the patent literature. Polyester films, coated with various kinds of pressure sensitive adhesives, have been laminated to image-bearing surfaces to protect the image. For example, U.S. Pat. No. 4,337,107 discloses laminating a film of cured epoxy-silane to produce a protective film exhibiting superior abrasion resistant properties. Because of their thickness (total of polyester base and adhesive layer), the laminating films cause optical distortion and hence loss of resolution. In addition, air is often entrapped in the soft pressure-sensitive adhesive during the laminating process, causing the finished product to contain undesirable air bubbles. Thinner protective coatings can be obtained, and with the exclusion of air pockets, by coating the surfaces of photographic elements with liquid compositions. After application, the thin liquid coating is hardened to yield the desired protective coat. Epoxy-silanes are especially useful in such coatings because of their resistance to abrasion; epoxy-silanes have the additional benefit of adhering well to silver halide emulsions and to other substrates. Examples of photographic coatings based on epoxy-silane compounds may be found in U.S. Pat. Nos. 4,156,046, 4,293,606 and 4,337,107.
Acrylate esters, and in particular the polyurethane acrylates, also offer good resistance to abrasion. Compositions based on acrylates cure rapidly and produce coatings of excellent clarity. Photographic applications of acrylics for protective overcoats are described in U.S. Pat. Nos. 4,333,998, 4,353,980, and 4,399,192.
In all cases, overcoats on photographic products must meet exacting requirements with respect to transparency, flexibility, adhesion, and thickness. But, in the case of phototools, even stricter requirements must be met. For example, the commercial product based on U.S. Pat. No. 4,077,830 provides a laminating film which produces an overcoat ranging in thickness from 25 to 51 micrometers (1 to 2 mils); such thicknesses limit optical resolutions to about 1 lp/457 micrometers (1 lp/18 mil) [this corresponds to a set of two parallel lines and a space between them, each line and space having the same width, fitting into a width of 18 mils]. This severely limits the application because phototools often require resolutions greater than one line pair/457 micrometers. In addition, the thickness of the overlay causes parallax during contact printing and this results in poor wall quality in the final printed circuit. While the above mentioned epoxy-silanes adhere well to silver halide emulsions, compositions described in the patents readily attack diazo emulsions. When the liquid compositions are applied to the imaged surface of a diazo phototool, the liquids aggressively attach the diazo emulsion and dissolve the image. The compositions are too efficient a solvent to be coated on a diazo phototool. While the acrylate compositions of the above patents are not nearly as aggressive towards the diazo emulsions as are the epoxy-silanes, they suffer from the disadvantage of not adhering to silver halide emulsions and to many other substrates. Since some phototools use silver halide emulsions, the acrylates, per se, would be unsuitable as a protective overcoat. While the above mentioned systems are useful for their specified applications, they fall short of the damanding requirements for use on phototools. The tapes are too thick and cause optical distortions, and their soft adhesives entrap air during lamination. The epoxy-silanes aggressively attack the diazo emulsions and dissolve the images they are suppose to protect. The acrylates exhibit poor adhesion to silver halide type emulsions.
Coating compositions which contain both a component whose polymerization is initiated by a free-radical catalyst and a component whose polymerization is cationically initiated, and wherein both components polymerize simultaneously via separate mechanisms, are known as a "mixed" or "hybrid" system. For example, acrylic systems that copolymerize with epoxy systems using two different photoinitiating catalysts are described in U.S. Pat. Nos. 4,025,348 and 4,156,035. U.S. Pat. No. 3,753,755 describes a mixed system of acrylic and epoxy compounds which is polymerized by heat alone, while U.S. Pat. No. 4,025,407 describes an acrylic and epoxy system which is cured by a combination of heat and light. U.S. Pat. No. 4,348,462 claims a mixed system of acrylics and silanes. U.S. Pat. No. 4,426,431 reports a mixed system of acrylics, epoxies, and silanes which are photopolymerized by two different photoinitiators and said to be useful as a restorative and protective treatment for photographic elements.